1. Field of the Invention
The invention relates to demodulators for pulse width modulated signals and, in particular, to such demodulators for measuring the pulse width of signals with variable pulse frequency.
2. Description of Related Art
The transmission of information using pulse width modulation at a constant pulse frequency is well known. In such systems a signal comprises a series of pulses at a constant frequency, the "high" or "on" period of the pulse, and hence the width, providing information, for example the sampled amplitude of an analog signal. The information can be represented by the "duty ratio", that is, the ratio of the "on" period of the pulse to the sum of the "on" period and the "off" period. The sum of the "on" and "off" periods is the constant period of the signal.
It is further known that information may be conveyed in a variable frequency pulse width modulated signal, that is, where the period of the signal is not constant. Attempts have been made to carry out measurements of such signals using analog techniques, for example those using a low-pass filter, wherein the average DC voltage is measured. Such systems necessarily require costly analog components as well as special calibration.